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WO2023098308A1 - A genetically engineered bacterium and its application in the preparation of sialyllactose - Google Patents

A genetically engineered bacterium and its application in the preparation of sialyllactose Download PDF

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Publication number
WO2023098308A1
WO2023098308A1 PCT/CN2022/124823 CN2022124823W WO2023098308A1 WO 2023098308 A1 WO2023098308 A1 WO 2023098308A1 CN 2022124823 W CN2022124823 W CN 2022124823W WO 2023098308 A1 WO2023098308 A1 WO 2023098308A1
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Prior art keywords
gene
genetically engineered
encoding
engineered bacterium
sialyltransferase
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PCT/CN2022/124823
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French (fr)
Inventor
Yan Wu
Yurou LIU
Jing Tang
Hong Xu
Shu Wang
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Synaura Biotechnology Shanghai Co Ltd
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Synaura Biotechnology Shanghai Co Ltd
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Priority to US18/576,640 priority Critical patent/US20240309416A1/en
Priority to EP22799841.6A priority patent/EP4344401A1/en
Publication of WO2023098308A1 publication Critical patent/WO2023098308A1/en
Anticipated expiration legal-status Critical
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Definitions

  • the invention belongs to the field of bioengineering, in particular to a genetically engineered bacterium and its application in the preparation of sialyllactose.
  • HMOs Human milk oligosaccharides
  • SHMOs sialylated HMOs
  • sialic acid always plays an important role in the rapid brain growth at the infant early development stage and the development process of immune system.
  • the content of sialic acids in infant formulas currently on the market is low. Therefore, it is necessary to provide sialic acids of sufficient quality and quantity to supplement the infant formulas and other nutritional compositions.
  • SA sialyllactose
  • CN111133112A disclosed that a Neu5Ac-producing strain is obtained by a method comprising inactivation of lacZ and araA genes, deletion of N-acetylneuraminic acid catabolism gene clusters nagAB and nanAEKT, deletion of wzxC-wcaJ genes, deletion of fucl and fucK genes, and integration of galETKM, lacY, cscBKAR, glmS, glmM, glmU, Gna1, slr1975, neuBC, andppsA into Escherichia coli genome, and is used for the screening of sialyltransferase. Some sialyltransferases with sialyltransferase activity are screened out, and can be used for the preparation of 3’-SL and 6’-SL. However, the resulting yield is not high because these genes are all integrated into the genome.
  • the technical problem to be solved by the present invention is to provide a genetically engineered bacterium and its application in the preparation of sialyllactose in order to overcome the defects of low yield and high production cost of the synthesis of sialyllactose in the prior art.
  • Using the genetically engineered bacterium of the present invention to produce sialyllactose has the advantages of high yield and low overall cost.
  • the inventors creatively knocked out genes related to the intracellular degradation of N-acetylneuraminic acid (Neu5Ac) in the wild-type strain BL2l (DE3) by CRISPR/Cas9 technology, thereby constructing a non-naturally occurring Escherichia coli strain for the production of Neu5Ac, in which an Neu5Ac or SL synthetic pathway containing at least one heterologous enzyme is constructed and the naturally occurring N-acetylneuraminic acid (Neu5Ac) catabolic pathway is selectively disabled.
  • the resulting genetically engineered bacterium can synthesize Neu5Ac or SL by means of self-metabolism using a single cheap exogenous carbon source in the fermentation broth.
  • the present invention solves the above technical problems mainly through the following technical solutions.
  • the first technical solution of the present invention is: a genetically engineered bacterium having an N-acetylneuraminic acid biosynthesis pathway, wherein the genetically engineered bacterium contains multiple copies of a gene neuB for coding a sialic acid synthase, and the gene neuB is initiated for expression by a strong promoter.
  • an N-acetylneuraminic acid catabolic pathway in the genetically engineered bacterium is disabled.
  • the disabled pathway may be conventional in the art, and in the present invention, the purpose of the disability is preferably achieved by knocking out all or part of the genes in the N-acetylneuraminic acid catabolism pathway.
  • Said all or part of the genes may be one or more of a gene nanK encoding N-acetylmannosamine kinase, a gene nanE encoding N-acetylmannosamine-6-phosphate epimerase, and a gene nanA encoding N-acetylneuraminic acid aldolase.
  • a gene LacZ encoding the lactose operon ⁇ -galactosidase in the genetically engineered bacterium may also be knocked out.
  • the multiple copies described in the first technical solution may be achieved by conventional means in the art, for example, by one or more means of replicon, multi-site integration in genome and insertion of an exogenous plasmid.
  • the multiple copies are achieved by means of insertion of an exogenous plasmid.
  • the strong promoter described in the first technical solution may be conventional in the art, preferably is a Tet promoter.
  • the genetically engineered bacterium described in the present invention preferably further comprises a gene neuB encoding sialic acid synthase, a gene slr1975 encoding N-acetylglucosamine 2-epimerase, a gene YqaB encoding N-acetylglucosamine-6-phosphate phosphatase, a gene Gna1 encoding glucosamine-6-phosphate acetyltransferase, a gene glmS encoding L-glutamine-D-fructose-6-phosphate transaminase, and a gene ppsA encoding phosphoenolpyruvate synthase.
  • the gene neuB, the gene slr1975, the gene YqaB, the gene Gna1, the gene glmS and the geneppsA are linked in tandem in a plasmid vector 1.
  • the genes in the plasmid vector 1 preferably meet one or more of the following conditionas that:
  • GenBank accession number of the gene neuB is AF305571
  • GenBank accession number of the gene slr1975 is BAL35720;
  • the gene YqaB is from the BL21 genome
  • GenBank accession number of the gene Gna1 is NP_116637;
  • nucleic acid sequence of the gene glmS is set forth in SEQ ID NO: 66;
  • the geneppsA is from the BL21 genome.
  • the backbone of the plasmid vector 1 in the present invention is preferably pACYCDuet.
  • the genetically engineered bacterium described in the present invention preferably further comprises a gene encoding N-acylneuraminate cytidylyltransferase and a gene encoding sialyltransferase; the two genes are preferably linked in tandem in a plasmid vector 2.
  • the N-acylneuraminate cytidylyltransferase is preferably the N-acylneuraminate cytidylyltransferase with NCBI accession number WP_003512903.1, and preferably comprises the nucleic acid sequence set forth in SEQ ID NO: 61.
  • the sialyltransferase in the present invention may be conventional in the art, preferably is ⁇ -2, 6-sialyltransferase or ⁇ -2, 3-sialyltransferase.
  • the ⁇ -2, 6-sialyltransferase is preferably an enzyme with NCBI accession number BAF91416.1, and preferably comprises the nucleic acid sequence set forth in SEQ ID NO: 62.
  • the ⁇ -2, 3-sialyltransferase is preferably an enzyme with NCBI accession number AJC62560.1, and preferably comprises the nucleic acid sequence set forth in SEQ ID NO: 63.
  • the backbone of the plasmid vector 2 in the present invention is preferably pET28a.
  • the starting bacterium of the genetically engineered bacterium is preferablyEscherichia coli BL21 (DE3) .
  • the second technical solution of the present invention is: a method for producing N-acetylneuraminic acid, comprising: culturing the genetically engineered bacterium as described in the first technical solution.
  • the third technical solution of the present invention is: a method for producing sialyllactose by fermentation, comprising: using the genetically engineered bacterium as described in the first technical solution, adding lactose into a fermentation medium for fermentation, and extracting the sialyllactose from the fermentation broth.
  • 3’-sialyllactose is obtained when the genetically engineered bacterium comprises a ⁇ -2, 3-sialyltransferase gene; 6’-sialyllactose is obtained when the genetically engineered bacterium comprises ⁇ -2, 6-sialyltransferase gene.
  • the fermentation medium in the present invention is preferably a TB medium which comprises 12 g/L of trypsin, 24 g/L of yeast extract, 4 mL/L of glycerol, 2.31 g/L of KH 2 PO 4 and 12.54 g/L of K 2 HPO 4 .
  • culture is induced with IPTG when cultured to an OD value of 0.6 to 0.8.
  • the trace element stock solution preferably comprises 54.4 g/L Ferric ammonium citrate, 9.8 g/L MnCl 2 ⁇ 4H 2 O, 1.6 g/L CoCl 2 ⁇ 6H 2 O, 1 g/L CuCl 2 ⁇ 2H 2 O, 1.9 g/L H 3 BO 3 , 9 g/L ZnSO 4 ⁇ 7H 2 O, 1.1 g/L Na 2 MoO 4 ⁇ 2H 2 O, 1.5 g/L Na 2 SeO 3 and 1.5 g/L NiSO 4 ⁇ 6H 2 O.
  • the fermentation culture conditions are preferably culture at 30°C with shaking at 250 rpm.
  • the reagents and raw materials used in the present invention are all commercially available.
  • the genetically engineered bacterium obtained in the present invention can obtain a higher yield of sialyllactose, and the yield of sialyllactose obtained after 24 hours of shaking flask fermentation can reach 2.5 to 3 g/L; on the other hand, during the fermentation, the genetically engineered bacterium can synthesize sialyllactose by means of self-metabolism using a single cheap exogenous carbon source.
  • Figure 1 shows the mapping of lacZ-knockout validation.
  • Figure 2 shows the mapping of pTargetF plasmid.
  • Figure 3 shows the mapping of the expression plasmid SL006.
  • Figure 4 shows the mapping of pET28a-neuB plasmid.
  • Figure 5 shows the mapping of pTac-neuB plasmid.
  • Figure 6 shows the mapping of pTet-neuB plasmid.
  • Figure 7 shows the mapping of SL023 plasmid.
  • Figure 8 shows the mapping of SL023-Tet plasmid.
  • Figure 9 shows the mapping of SL023-Tac plasmid.
  • Figure 10 shows the HPLC detection profile of Neu5Ac in fermentation broth when the expression ofneuB was regulated by the Tet promoter.
  • Figure 11 shows the detection profile of 6’-SL standard, with the peak time of 17.818min.
  • Figure 12 shows the HPLC detection profile of 6’-SL in fermentation broth when the expression ofneuB was regulated by the Tet promoter.
  • Figure 13 shows the SL037 plasmid map.
  • Figure 14 shows the detection profile of 3’-SL standard, with the peak time of 15.049min.
  • Figure 15 shows the HPLC detection profile of 3’-SL in fermentation broth when the expression ofneuB was regulated by the Tet promoter.
  • BL21 (DE3) strain was purchased from Novagen Company, with the article number of 69450-M; Escherichia coli Trans 10 competent cell was purchased from Beijing TransGen Biotech Co., Ltd. ; plasmid extraction kit and gel recovery kit were purchased from Sangon Biotech (Shanghai) Co., Ltd., and the SDS-PAGE kit was purchased from Shanghai Yamei Biomedical Technology Co., Ltd.
  • sgRNA small guide RNA
  • the specific amplification of each fragment is performed with primers designed according to Table 3 (synthesized by Tsingke) using pTargetF plasmid (see Figure 2 for the mapping) or BL21 genome as a template, and high-fidelity enzyme Primer Star Mix from Takala was used for PCR reaction.
  • the reaction system was as follows:
  • the PCR amplification procedure was as follows:
  • the target fragments were recovered by gel cutting using a gel recovery kit.
  • the target fragments were ligated and recombined using a multi-fragment recombinase from NEB, and the ligation and recombination products were transformed into E. coli competent cells Trans 10. Sterilized LB liquid medium was added, shook and cultured with shaking at 250 rpm at 37°C for 1 h;
  • the bacterium liquid was subjected to PCR assay, and 500 ⁇ l of the positive bacteriuml liquid, which was verified to be positive, was sent to Tsingke Company for sequencing, and the remaining bacterium liquid was stored in 20%glycerol.
  • PCR validation was carried out on single colonies on the resistant plate.
  • the primers for validation are shown in Table 4, and the mapping of sequencing validation is shown in Figure 1.
  • the strain with LacZ gene knockout was obtained by verification;
  • strains with LacZ gene knockout were selected and cultured with shaking, and induced by adding rhamnose at a final concentration of 10 mM for the loss treatment of the sgRNA plasmid pTargetF- ⁇ LacZ;
  • the pTargetF- ⁇ LacZ plasmid was streaked to verify whether the loss occurred (see Table 4 for primers) , and the strain with sgRNA loss and LacZ gene knocked out was named as SLSI020.
  • Competent preparation and knockout operation of SLIS020 were the same as 1.2.1.
  • the pTargetF- ⁇ nanA plasmid was used to knock out the nanA gene, and the method was the same as 1.2.1, to obtain the strain with nanA gene knockout, which was named as SLIS024.
  • the SLIS026 strain was subjected to loss of pCas-SAC plasmid: the SLIS026 strain with sgRNA loss was inoculated on non-resistant LB plate, cultured at 42°C, and verified by PCR with pCas-SAC validation primers in Table 4 to ensure that the chassis strain SLIS026 without pCas-SAC plasmid was obtained.
  • the gene glmS* (GA010) is a mutant version of Escherichia coli L-glutamine-D-fructose-6-phosphate transaminase gene (Metab Eng., 2005 May; 7 (3) : 201-14) , and its nucleic acid sequence is set forth in SEQ ID NO: 66; Gna1 (GA009, GenBank: NP_116637) encodes the glucosamine-6-phosphate acetyltransferase from Saccharomyces cerevisiae; the gene slr1975 (GA006, GenBank: BAL35720) encodes Synechocystis sp.
  • PCC6803 N-acetylglucosamine 2-epimerase
  • the gene neuB (GA005, GenBank: AF305571) encodes the sialic acid synthase from Campylobacter jejuni
  • the gene ppsA encodes the phosphoenolpyruvate synthase of Escherichia coli BL21 (DE3)
  • the gene YqaB encodes the N-acetylglucosamine-6-phosphate phosphatase.
  • the slr1975, Gnal, glmS, neuB, and promoter Tet, Tac gene sequences were synthesized and ligated into the puc57 vector by Sangon Biotech (Shanghai) Co., Ltd., and YqaB and ppsA were all from the BL21 genome.
  • the amplified DNA fragments were recovered by gel cutting using a gel recovery kit, and positive single colonies were obtained by recombination, transformation, and plate screening using the multi-fragment recombination kit of NEB Company, and then the correct plasmids were obtained by picking up the colony, shaking bacteria, and verifying via sequencing by Tsingke Biotechnology Co., Ltd..
  • the sequence of the Tac promoter is set forth in SEQ ID NO: 64.
  • the sequence of the Tet promoter is set forth in SEQ ID NO: 65.
  • Competent cells were prepared on the basis of the gene knockout strain SLIS026, and the specific method was the same as 1.2.1. Then plasmid SL006 were transformed, alone, and together with pET28a-neuB, pTac-neuB, and pTet-neuB, into SLIS026 competent cells, and the correct clones were screened on LB plates (kanamycin 50 ⁇ g/mL, chloramphenicol 25 ⁇ g/mL) . Four strains E.
  • coli SLIS026-SA (SL006, SL006+pET28a-neuB, SL006+pTet-neuB, SL006+pTac-neuB) carrying Neu5Ac synthesis pathway were obtained correspondingly after validation by PCR.
  • (1) TB medium 12g of trypsin, 24g of yeast extract, 4mL of glycerol, 2.31g KH 2 PO 4 , 12.54g K 2 HPO 4 , supplemented to 1000mL with deionized water, and finally dispensed into conical flasks at 100mL/bottle and wrapped well, sterilized at 121 °C for 30 min, and stored at room temperature.
  • (2) LB medium 10 g of tryptone, 5 g of yeast extract, 10 g of NaCl, and 15 g of agar were weighed respectively, and were dissolved with distilled water and mixed well, and were supplemented to 1 L after the pH was adjusted to 7.2 with 1 mol/L NaOH, and finally divided into conical flasks and wrapped well, sterilized at 121 °C for 30 min, and stored at 4 °C.
  • the LB liquid was withour agar.
  • Trace element stock solution 54.4g/L ferric ammonium citrate, 9.8g/L MnCl 2 ⁇ 4H 2 O, 1.6g/L CoCl 2 ⁇ 6H 2 O, 1g/L CuCl 2 ⁇ 2H 2 O, 1.9g/L H 3 BO 3 , 9g/L ZnSO 4 ⁇ 7H 2 O, 1.1g/L Na 2 MoO 4 ⁇ 2H 2 O, 1.5g/L Na 2 SeO 3 , 1.5g/L NiSO 4 ⁇ 6H 2 O.
  • the trace element stock solution was prepared according to the above concentrations, sterilized at 121 °C for 30 min, and stored at 4 °C for future use.
  • 1000g/L glycerol 1000g glycerol was weighed, supplemented to 1L with deionized water, then sterilized at 121°C for 30 minutes, and stored at room temperature.
  • LB medium kanamycin 50 ⁇ g/mL, chloramphenicol 25 ⁇ g/mL
  • the amplified DNA fragments were recovered by gel cutting using a gel recovery kit, and positive single colonies were obtained by recombination, transformation, and plate screening using the multi-fragment recombination kit of NEB Company, and then the correct plasmids were obtained by picking up the colony, shaking bacteria, and verifying via sequencing by Tsingke Biotechnology Co., Ltd..
  • Competent cells were prepared on the basis of the gene knockout strain SLIS026, and the specific method was the same as 1.2.1. Then the following plasmid combinations: SL006+pET28a-neuB+SL023, SL006+pET28a-neuB+SL023-Tet, SL006+pET28a-neuB+SL023-Tac, SL006+pTet-neuB+SL023, SL006+pTet-neuB+SL023-Tac, SL006+pTet-neuB+SL023-Tet ⁇ SL006+pTac-neuB+SL023, SL006+pTac-neuB+SL023-Tac, SL006+pTac-neuB+SL023-Tet were transformed into SLIS026 competent cells, respectively, and the correct clones were screened on LB plates (kanamycin 50 ⁇ g/mL, chloramphenicol
  • (1) TB medium 12g of trypsin, 24g of yeast extract, 4mL of glycerol, 2.31g KH 2 PO 4 , 12.54g K 2 HPO 4 , supplemented to 1000mL with deionized water, and finally divided into conical flasks at 100mL/bottle and wrapped well, sterilized at 121 °C for 30 min, and stored at room temperature.
  • (2) LB medium 10 g of tryptone, 5 g of yeast extract, 10 g of NaCl, and 15 g of agar were weighed respectively, dissolved with distilled water and mixed well, and were supplemented to 1 L after the pH was adjusted to 7.2 with 1 mol/L NaOH, and finally divided into conical flasks and wrapped well, sterilized at 121 °C for 30 min, and stored at 4 °C.
  • the LB liquid was withour agar.
  • Trace element stock solution 54.4g/L ferric ammonium citrate, 9.8 g/L MnCl 2 ⁇ 4H 2 O, 1.6 g/L CoCl 2 ⁇ 6H 2 O, 1 g/L CuCl 2 ⁇ 2H 2 O, 1.9 g/L H 3 BO 3 , 9 g/L ZnSO 4 ⁇ 7H 2 O, 1.1 g/L Na 2 MoO 4 ⁇ 2H 2 O, 1.5 g/L Na 2 SeO 3 , 1.5 g/L NiSO 4 ⁇ 6H 2 O.
  • the trace element stock solution was prepared according to the above concentrations, sterilized at 121 °C for 30 min, and stored at 4 °C for future use.
  • 1000 g/L glycerol 1000g glycerol was weighed, supplemented to 1L with deionized water, then sterilized at 121°C for 30 minutes, and stored at room temperature.
  • LB medium kanamycin 50 ⁇ g/mL, chloramphenicol 25 ⁇ g/mL
  • the amplified DNA fragments were recovered by gel cutting using a gel recovery kit, and positive single colonies were obtained by recombination, transformation, and plate screening using the multi-fragment recombination kit of NEB Company, and then the correct plasmids were obtained by picking up the colony, shaking bacteria, and verifying via sequencing by Tsingke Biotechnology Co., Ltd..
  • Competent cells were prepared on the basis of the gene knockout strain SLIS026, the specific method was the same as 1.2.1, and then the plasmid combination SL006+pTet-neuB+SL037 was transformed into SLIS026 competent cells with reference to the plasmid combination with the best 6’-SL fermentation result, and the correct clones were screened on LB plates (kanamycin 50 ⁇ g/mL, chloramphenicol 25 ⁇ g/mL, ampicillin 100 ⁇ g/mL) to obtain a strain E. coli SLIS026-3SL carrying the 3’-SL synthesis pathway.
  • Means of sample treatment Means of sample treatment were the same as 3.2. Quantitative detection of 3’-SL was carried out, and the results showed that the content of 3’-SL can reach 3g/L at 24 hours of fermentation.
  • Figure 14 shows the detection profile of 3’-SL standard.
  • Figure 15 shows the detection profile of 3’-SL fermentation broth.

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Abstract

The invention discloses a genetically engineered bacterium and its application in the preparation of sialyllactose. The genetically engineered bacterium has an N-acetylneuraminic acid biosynthesis pathway, comprises multiple copies of a gene neuB for encoding sialic acid synthase, and the gene neuB is initiated for expression by a strong promoter. Using the genetically engineered bacteria of the invention to produce sialyllactose has the advantages of high yield and low overall cost.

Description

A GENETICALLY ENGINEERED BACTERIUM AND ITS APPLICATION IN THE PREPARATION OF SIALYLLACTOSE Technical Field
The invention belongs to the field of bioengineering, in particular to a genetically engineered bacterium and its application in the preparation of sialyllactose.
Background of the Invention
Human milk oligosaccharides (HMOs) are one of the components with high nutritional value in human milk. According to the characteristics of the monosaccharide composition and structure, HMOs can be classified into neutral fucosyl, neutral non-fucosyl, and sialic acid, etc. Among them, the proportion of sialylated HMOs (SHMOs) is higher than 20%, and the species of the sialylated HMOs are relatively abundant. 3’-Sialyllactose (3’-SL) , 6’-Sialyllactose, (6’-SL) , and Sialyllacto-N-tetraose a (LST-a) , etc., which are abundant in HMOs, have functions such as immune regulation, helping brain development and regulation of gut microbiota. Therefore, sialic acid always plays an important role in the rapid brain growth at the infant early development stage and the development process of immune system. However, the content of sialic acids in infant formulas currently on the market is low. Therefore, it is necessary to provide sialic acids of sufficient quality and quantity to supplement the infant formulas and other nutritional compositions.
At present, there are chemical and enzymatic synthesis, biosynthesis and other methods for the production of sialyllactose (SL) . However, there are many difficulties in the actual production process of chemical synthesis or enzymatic synthesis, for example, stereochemical control, formation of specific linking, availability of raw materials, etc. The method through microbial metabolic synthesis using synthetic biology technology is more economical and efficient compared with chemical synthesis and enzymatic synthesis. Studies have shown  that sialic acid (SA) can be synthesized by biosynthesis using glucose as a substrate, and the most abundant sialic acid in the products of this method seems to be N-acetylneuraminic acid (NANA, NeuNAc, Neu5Ac) . This method is simple, has low cost, and produces considerable yield.
CN111133112A disclosed that a Neu5Ac-producing strain is obtained by a method comprising inactivation of lacZ and araA genes, deletion of N-acetylneuraminic acid catabolism gene clusters nagAB and nanAEKT, deletion of wzxC-wcaJ genes, deletion of fucl and fucK genes, and integration of galETKM, lacY, cscBKAR, glmS, glmM, glmU, Gna1, slr1975, neuBC, andppsA into Escherichia coli genome, and is used for the screening of sialyltransferase. Some sialyltransferases with sialyltransferase activity are screened out, and can be used for the preparation of 3’-SL and 6’-SL. However, the resulting yield is not high because these genes are all integrated into the genome.
Summary of the Invention
The technical problem to be solved by the present invention is to provide a genetically engineered bacterium and its application in the preparation of sialyllactose in order to overcome the defects of low yield and high production cost of the synthesis of sialyllactose in the prior art. Using the genetically engineered bacterium of the present invention to produce sialyllactose has the advantages of high yield and low overall cost.
The inventors creatively knocked out genes related to the intracellular degradation of N-acetylneuraminic acid (Neu5Ac) in the wild-type strain BL2l (DE3) by CRISPR/Cas9 technology, thereby constructing a non-naturally occurring Escherichia coli strain for the production of Neu5Ac, in which an Neu5Ac or SL synthetic pathway containing at least one heterologous enzyme is constructed and the naturally occurring N-acetylneuraminic acid (Neu5Ac) catabolic pathway is selectively disabled. The resulting genetically engineered bacterium can synthesize Neu5Ac or SL by means of self-metabolism using a single cheap exogenous carbon source in the fermentation broth.
The present invention solves the above technical problems mainly through the following technical solutions.
The first technical solution of the present invention is: a genetically engineered bacterium having an N-acetylneuraminic acid biosynthesis pathway, wherein the genetically engineered bacterium contains multiple copies of a gene neuB for coding a sialic acid synthase, and the gene neuB is initiated for expression by a strong promoter.
Preferably, an N-acetylneuraminic acid catabolic pathway in the genetically engineered bacterium is disabled. The disabled pathway may be conventional in the art, and in the present invention, the purpose of the disability is preferably achieved by knocking out all or part of the genes in the N-acetylneuraminic acid catabolism pathway.
Said all or part of the genes may be one or more of a gene nanK encoding N-acetylmannosamine kinase, a gene nanE encoding N-acetylmannosamine-6-phosphate epimerase, and a gene nanA encoding N-acetylneuraminic acid aldolase.
Further, a gene LacZ encoding the lactose operonβ-galactosidase in the genetically engineered bacterium may also be knocked out.
The multiple copies described in the first technical solution may be achieved by conventional means in the art, for example, by one or more means of replicon, multi-site integration in genome and insertion of an exogenous plasmid. In a preferred embodiment of the present invention, the multiple copies are achieved by means of insertion of an exogenous plasmid.
The strong promoter described in the first technical solution may be conventional in the art, preferably is a Tet promoter.
The genetically engineered bacterium described in the present invention preferably further comprises a gene neuB encoding sialic acid synthase, a gene slr1975 encoding N-acetylglucosamine 2-epimerase, a gene YqaB encoding N-acetylglucosamine-6-phosphate phosphatase, a gene Gna1 encoding glucosamine-6-phosphate acetyltransferase, a gene glmS encoding L-glutamine-D-fructose-6-phosphate transaminase, and a gene ppsA encoding  phosphoenolpyruvate synthase.
Preferably, the gene neuB, the gene slr1975, the gene YqaB, the gene Gna1, the gene glmS and the geneppsA are linked in tandem in a plasmid vector 1.
Among these, the genes in the plasmid vector 1 preferably meet one or more of the following conditionas that:
the GenBank accession number of the gene neuB is AF305571;
the GenBank accession number of the gene slr1975 is BAL35720;
the gene YqaB is from the BL21 genome;
the GenBank accession number of the gene Gna1 is NP_116637;
the nucleic acid sequence of the gene glmS is set forth in SEQ ID NO: 66;
the geneppsA is from the BL21 genome.
The backbone of the plasmid vector 1 in the present invention is preferably pACYCDuet.
The genetically engineered bacterium described in the present invention preferably further comprises a gene encoding N-acylneuraminate cytidylyltransferase and a gene encoding sialyltransferase; the two genes are preferably linked in tandem in a plasmid vector 2.
Wherein, the N-acylneuraminate cytidylyltransferase is preferably the N-acylneuraminate cytidylyltransferase with NCBI accession number WP_003512903.1, and preferably comprises the nucleic acid sequence set forth in SEQ ID NO: 61.
The sialyltransferase in the present invention may be conventional in the art, preferably is α-2, 6-sialyltransferase or α-2, 3-sialyltransferase. Wherein, the α-2, 6-sialyltransferase is preferably an enzyme with NCBI accession number BAF91416.1, and preferably comprises the nucleic acid sequence set forth in SEQ ID NO: 62. The α-2, 3-sialyltransferase is preferably an enzyme with NCBI accession number AJC62560.1, and preferably comprises the nucleic acid sequence set forth in SEQ ID NO: 63.
The backbone of the plasmid vector 2 in the present invention is preferably pET28a.
In the present invention, the starting bacterium of the genetically engineered  bacterium is preferablyEscherichia coli BL21 (DE3) .
The second technical solution of the present invention is: a method for producing N-acetylneuraminic acid, comprising: culturing the genetically engineered bacterium as described in the first technical solution.
The third technical solution of the present invention is: a method for producing sialyllactose by fermentation, comprising: using the genetically engineered bacterium as described in the first technical solution, adding lactose into a fermentation medium for fermentation, and extracting the sialyllactose from the fermentation broth.
Optionally, 3’-sialyllactose is obtained when the genetically engineered bacterium comprises a α-2, 3-sialyltransferase gene; 6’-sialyllactose is obtained when the genetically engineered bacterium comprises α-2, 6-sialyltransferase gene.
The fermentation medium in the present invention is preferably a TB medium which comprises 12 g/L of trypsin, 24 g/L of yeast extract, 4 mL/L of glycerol, 2.31 g/L of KH 2PO 4 and 12.54 g/L of K 2HPO 4.
In the method described in the third technical solution, preferably, culture is induced with IPTG when cultured to an OD value of 0.6 to 0.8.
In addition, 2 g/L MgSO 4·7H 2O, 20 g/L glycerol, 1 mL/L trace element stock solution and 5g/L lactose are supplemented after the induction culture; the trace element stock solution preferably comprises 54.4 g/L Ferric ammonium citrate, 9.8 g/L MnCl 2·4H 2O, 1.6 g/L CoCl 2·6H 2O, 1 g/L CuCl 2·2H 2O, 1.9 g/L H 3BO 3, 9 g/L ZnSO 4·7H 2O, 1.1 g/L Na 2MoO 4·2H 2O, 1.5 g/L Na 2SeO 3 and 1.5 g/L NiSO 4·6H 2O.
In the present invention, the fermentation culture conditions are preferably culture at 30℃ with shaking at 250 rpm.
The numbers following the terms in the present invention, such as "1" and "2" in plasmid vector 1 and plasmid vector 2, have no actual meaning, but only distinguish the same terms.
On the basis of conforming to common knowledge in the art, the above preferred conditions can be combined arbitrarily to obtain preferred examples of  the present invention.
The reagents and raw materials used in the present invention are all commercially available.
The positive progressive effects of the present invention lie in:
On the one hand, the genetically engineered bacterium obtained in the present invention can obtain a higher yield of sialyllactose, and the yield of sialyllactose obtained after 24 hours of shaking flask fermentation can reach 2.5 to 3 g/L; on the other hand, during the fermentation, the genetically engineered bacterium can synthesize sialyllactose by means of self-metabolism using a single cheap exogenous carbon source.
Description of the Drawings
Figure 1 shows the mapping of lacZ-knockout validation.
Figure 2 shows the mapping of pTargetF plasmid.
Figure 3 shows the mapping of the expression plasmid SL006.
Figure 4 shows the mapping of pET28a-neuB plasmid.
Figure 5 shows the mapping of pTac-neuB plasmid.
Figure 6 shows the mapping of pTet-neuB plasmid.
Figure 7 shows the mapping of SL023 plasmid.
Figure 8 shows the mapping of SL023-Tet plasmid.
Figure 9 shows the mapping of SL023-Tac plasmid.
Figure 10 shows the HPLC detection profile of Neu5Ac in fermentation broth when the expression ofneuB was regulated by the Tet promoter.
Figure 11 shows the detection profile of 6’-SL standard, with the peak time of 17.818min.
Figure 12 shows the HPLC detection profile of 6’-SL in fermentation broth when the expression ofneuB was regulated by the Tet promoter.
Figure 13 shows the SL037 plasmid map.
Figure 14 shows the detection profile of 3’-SL standard, with the peak time of 15.049min.
Figure 15 shows the HPLC detection profile of 3’-SL in fermentation broth  when the expression ofneuB was regulated by the Tet promoter.
Examples
The present invention is further illustrated by the Examples below, but the present invention is not limited to the scope of the described Examples.
In order to further illustrate the technical means adopted in the present invention and its effects, they are described in detail in combination with the figures and the preferred embodiments of the present invention. The experimental methods in which the specific conditions are not indicated in the following examples are selected according to conventional methods and conditions, or according to the product instruction.
BL21 (DE3) strain was purchased from Novagen Company, with the article number of 69450-M; Escherichia coli Trans 10 competent cell was purchased from Beijing TransGen Biotech Co., Ltd. ; plasmid extraction kit and gel recovery kit were purchased from Sangon Biotech (Shanghai) Co., Ltd., and the SDS-PAGE kit was purchased from Shanghai Yamei Biomedical Technology Co., Ltd.
HPLC detection method of sialyllactose: Chromatographic column: Sepax HP-Amide (250 x 4.6mm, 5um) . Buffer salts: 10 mM ammonium formate (pH 3.0) . Mobile phase: acetonitrile: buffer salts = 70: 30. Flow rate: 1.0 mL/min; concentration: 1 mg/mL; detection wavelength: 210 nm; injection volume: 10 μl; column temperature: 35℃.
Example 1 Construction of basic strain SLIS026
1.1 Construction of a plasmid comprising the small guide RNA (sgRNA) required for the CRISPR/Cas9 knockout system
The specific amplification of each fragment is performed with primers designed according to Table 3 (synthesized by Tsingke) using pTargetF plasmid (see Figure 2 for the mapping) or BL21 genome as a template, and high-fidelity enzyme Primer Star Mix from Takala was used for PCR reaction. The reaction system was as follows:
Table 1 PCR amplification reaction system
Figure PCTCN2022124823-appb-000001
The PCR amplification procedure was as follows:
Table 2 PCR reaction program
Figure PCTCN2022124823-appb-000002
5 μl of the amplified product was subjected to 1%agarose electrophoresis to detect the amplification result. The target fragments were recovered by gel cutting using a gel recovery kit. The target fragments were ligated and recombined using a multi-fragment recombinase from NEB, and the ligation and recombination products were transformed into E. coli competent cells Trans 10. Sterilized LB liquid medium was added, shook and cultured with shaking at 250 rpm at 37℃ for 1 h;
(2) Spots were picked to the LB solid plate with spectinomycin added in advance, and the plate was inverted overnight at 37℃;
(3) After white single colonies have grown, the white single colonies was picked into centrifuge tubes containing 2 mL of LB liquid medium (containing 50 μg/mL spectinomycin) , and were cultured with shaking at 180 rpm for 6 hours at 37 ℃;
(4) The bacterium liquid was subjected to PCR assay, and 500 μl of the positive bacteriuml liquid, which was verified to be positive, was sent to Tsingke Company for sequencing, and the remaining bacterium liquid was stored in 20%glycerol.
(5) The strains which were verified by sequencing to be correct, were expanded, and plasmid extraction was carried out by using a plasmid extraction kit from Sangon. The sgRNA plasmids comprising the BL21 genome were obtained, and named as pTargetF-ΔLacZ, pTargetF-ΔnanKE, and pTargetF-ΔnanA, respectively.
Table 3 Primer information for construction of lacZ, nanAKE-knockout sgRNA plasmid
Figure PCTCN2022124823-appb-000003
Figure PCTCN2022124823-appb-000004
1.2 gene knockout of lacZ and nanAKE
1.2.1 BL21 strain lacZ (GA001) gene knockout
(1) Preparation of BL21 competent cells: the strain BL21 stored at -80 ℃was streaked and cultured to obtain single colonies; the single colonies were picked up and inoculated in 5 mL of LB medium, and cultured with shaking at 200 rpm at 37 ℃ until the OD reaches about 0.5 (for about 3h) , then the culture was ice-bathed for 30min; the bacterial broth was transferred to a pre-cooled sterile centrifuge tube and centrifuged at 4000rpm for 10min at 4℃, then the supernatant was discarded, and the bacteria were collected; the bacteria were resuspended with pre-cooled sterile water and centrifuged at 4000rpm for 10min at 4℃, then the supernatant was discarded; the bacteria were resuspended with a solution containing 0.1M CaCl 2 twice, and centrifuged at 4000rpm for 10min at 4℃, then the supernatant was discarded; finally, the cells were resuspended with appropriate amount of 15%glycerol in 0.1M CaCl 2 solution, divided into 1.5mL centrifuge tubes with 100μl per tube, rapidly frozen in liquid nitrogen, and stored at -80℃.
(2) 3 μl pCas-sac plasmid was added to 100 μL E. coli BL21 competent, placed on ice for 30 min, then heat-shocked at 42 ℃ for 45 s, and immediately  placed on ice for 2-5 min; placed on a shaker at 30 ℃ and incubated for 45 min after adding 800 μL LB, spread onto a plate (Km resistant, LB medium) , inverted in an incubator at 30 ℃, and cultured overnight; the colony was picked out into LB medium (Kana resistant) , and cultured for several hours followed by bacteria preservation (30%glycerol final concentration) .
(3) The pCas-sac/BL21 transformants were picked and inoculated into LB sieve tubes (Kara-resistant) , cultured at 30℃ until the OD reaches 0.2, and arabinose was added at a final concentration of 2 g/L for induction, and competent preparation was performed when the OD reaches 0.4, the preparation method was the same as operation (1) ;
(4) The correctly constructed pTargetF-△LacZ plasmid was transformed into pCas-sac/BL21 competent cells by heat shock method, spread onto LB plates (k+, spe+) after recovery, and cultured at 30℃ overnight;
(5) PCR validation was carried out on single colonies on the resistant plate. The primers for validation are shown in Table 4, and the mapping of sequencing validation is shown in Figure 1. The strain with LacZ gene knockout was obtained by verification;
(6) The strains with LacZ gene knockout were selected and cultured with shaking, and induced by adding rhamnose at a final concentration of 10 mM for the loss treatment of the sgRNA plasmid pTargetF-△LacZ;
(7) The pTargetF-△LacZ plasmid was streaked to verify whether the loss occurred (see Table 4 for primers) , and the strain with sgRNA loss and LacZ gene knocked out was named as SLSI020.
1.2.2 Knockout of Neu5Ac degradation related gene nanKAE gene based on SLIS020 strain
(1) Competent preparation and knockout operation of SLIS020 were the same as 1.2.1. The pTargetF-△nanA plasmid was used to knock out the nanA gene, and the method was the same as 1.2.1, to obtain the strain with nanA gene knockout, which was named as SLIS024.
(2) Knockout of nanKE gene was performed on SLIS024 strain, using pTargetF-△nanE plasmid for knockout, the method was the same as 1.2.1, and the  strain with nanKE gene knockout completed was named as SLIS026.
(3) The SLIS026 strain was subjected to loss of sgRNA plasmid, and the method was the same as 1.2.1.
(4) The SLIS026 strain was subjected to loss of pCas-SAC plasmid: the SLIS026 strain with sgRNA loss was inoculated on non-resistant LB plate, cultured at 42℃, and verified by PCR with pCas-SAC validation primers in Table 4 to ensure that the chassis strain SLIS026 without pCas-SAC plasmid was obtained.
Table 4 Primers for validation of LacZ, nanAKE and other gene knockouts
Figure PCTCN2022124823-appb-000005
Example 2 Construction of expression plasmid related to N-acetylneuraminic acid (Neu5Ac) synthesis and preparation of Neu5Ac
2.1 Construction of expression plasmid for Neu5Ac synthesis
(1) The gene glmS* (GA010) is a mutant version of Escherichia coli L-glutamine-D-fructose-6-phosphate transaminase gene (Metab Eng., 2005 May; 7 (3) : 201-14) , and its nucleic acid sequence is set forth in SEQ ID NO: 66; Gna1 (GA009, GenBank: NP_116637) encodes the glucosamine-6-phosphate acetyltransferase from Saccharomyces cerevisiae; the gene slr1975 (GA006, GenBank: BAL35720) encodes Synechocystis sp. PCC6803 N-acetylglucosamine  2-epimerase; the gene neuB (GA005, GenBank: AF305571) encodes the sialic acid synthase from Campylobacter jejuni; the gene ppsA encodes the phosphoenolpyruvate synthase of Escherichia coli BL21 (DE3) ; the gene YqaB encodes the N-acetylglucosamine-6-phosphate phosphatase. The slr1975, Gnal, glmS, neuB, and promoter Tet, Tac gene sequences were synthesized and ligated into the puc57 vector by Sangon Biotech (Shanghai) Co., Ltd., and YqaB and ppsA were all from the BL21 genome.
(2) PCR amplification was performed according to the primers and templates listed in Table 5 to obtain the target fragments. The PCR reaction system and conditions are the same as 1.1 in Example 1.
(3) The amplified DNA fragments were recovered by gel cutting using a gel recovery kit, and positive single colonies were obtained by recombination, transformation, and plate screening using the multi-fragment recombination kit of NEB Company, and then the correct plasmids were obtained by picking up the colony, shaking bacteria, and verifying via sequencing by Tsingke Biotechnology Co., Ltd..
Table 5 Primers required for the plasmid construction of expression plasmids SL006, pET28a-neuB and others
Figure PCTCN2022124823-appb-000006
Figure PCTCN2022124823-appb-000007
(4) Promoter sequence:
The sequence of the Tac promoter is set forth in SEQ ID NO: 64.
The sequence of the Tet promoter is set forth in SEQ ID NO: 65.
2.2 Preparation of N-acetylneuraminic acid (Neu5Ac)
2.2.1 Construction of N-acetylneuraminic acid (Neu5Ac) -producing E. coli strains
Competent cells were prepared on the basis of the gene knockout strain SLIS026, and the specific method was the same as 1.2.1. Then plasmid SL006 were transformed, alone, and together with pET28a-neuB, pTac-neuB, and pTet-neuB, into SLIS026 competent cells, and the correct clones were screened  on LB plates (kanamycin 50 μg/mL, chloramphenicol 25 μg/mL) . Four strains E. coli SLIS026-SA (SL006, SL006+pET28a-neuB, SL006+pTet-neuB, SL006+pTac-neuB) carrying Neu5Ac synthesis pathway were obtained correspondingly after validation by PCR.
2.2.2 Production of N-acetylneuraminic acid (Neu5Ac) by SLIS026-Neu5Ac strain
(1) TB medium: 12g of trypsin, 24g of yeast extract, 4mL of glycerol, 2.31g KH 2PO 4, 12.54g K 2HPO 4, supplemented to 1000mL with deionized water, and finally dispensed into conical flasks at 100mL/bottle and wrapped well, sterilized at 121 ℃ for 30 min, and stored at room temperature.
(2) LB medium: 10 g of tryptone, 5 g of yeast extract, 10 g of NaCl, and 15 g of agar were weighed respectively, and were dissolved with distilled water and mixed well, and were supplemented to 1 L after the pH was adjusted to 7.2 with 1 mol/L NaOH, and finally divided into conical flasks and wrapped well, sterilized at 121 ℃ for 30 min, and stored at 4 ℃. The LB liquid was withour agar.
(3) 200g/L MgSO 4·7H 2O stock solution: 10g MgSO4·7H2O solution was weighed and dissolved into deionized water, supplemented to 50mL after completely dissolved, then sterilized at 121℃ for 30 minutes, and stored at room temperature.
(4) Trace element stock solution: 54.4g/L ferric ammonium citrate, 9.8g/L MnCl 2·4H 2O, 1.6g/L CoCl 2·6H 2O, 1g/L CuCl 2·2H 2O, 1.9g/L H 3BO 3, 9g/L ZnSO 4·7H 2O, 1.1g/L Na 2MoO 4·2H 2O, 1.5g/L Na 2SeO 3, 1.5g/L NiSO 4·6H 2O. The trace element stock solution was prepared according to the above concentrations, sterilized at 121 ℃ for 30 min, and stored at 4 ℃ for future use.
(5) 1000g/L glycerol: 1000g glycerol was weighed, supplemented to 1L with deionized water, then sterilized at 121℃ for 30 minutes, and stored at room temperature.
(6) The strains were inoculated into 5 mL of LB medium (kanamycin 50 μg/mL, chloramphenicol 25 μg/mL) , and cultured at 37℃ and 250 rpm for 4 h. Then the seed liquid was inoculated into fresh TB medium at a ratio of seed liquid: medium = 1: 100, cultured at 37 ℃, 250 rpm to OD600 = 0.6-0.8, and  IPTG was added to a final concentration of 0.1 mM and the medium was cultured under the condition of 25 ℃, 250 rpm for 15h for the inducible expression of protein.
(7) After the inducible expression of protein, 1mL of 200g/L MgSO 4·7H 2O (final concentration of 2g/L) , 1mL of trace element stock solution (final concentration 0.1%) , and 2mL of 1000g/L glycerol (final concentration of 20g/L) were added to the shaking flask, and cultured at 30 ℃ and 250 rpm for 24 h, and then sampled to detect the content of Neu5Ac.
(8) Means of sample treatment: the fermented bacterial solution was sterilized at 121 ℃ for 20 min, then centrifuged (4 ℃, 4000 rpm for 15 min) to remove the precipitation and retain the supernatant, and supplemented to 100mL with the sterilized TB medium for future use. 1mL of the solution was sampled and passed through a 0.22μm filter membrane, and detected with ion-pair ion. The strains using the Tet promoter were detected to have the highest Neu5Ac yields, 2.9g/L of Neu5Ac. Figure 10 shows the Neu5Ac detection profile.
Example 3 Construction of 6’-SL (6’-sialyllactose) synthesis-related expression plasmid and preparation of 6’-SL
3.1 Construction of 6’-SL synthesis plasmid
(1) Total gene synthesis was performed on the gene SEQ ID NO: 61 of N-acylneuraminate cytidylyltransferase CSS (accession number WP_003512903.1) (GA031) ; the gene SEQ ID NO: 62 of α-2, 6-2, 6-sialyltransferase (6ST, GA025) with accession number BAF91416.1, and both gene sequences were synthesized and ligated into the pET28a vector by Sangon Biotech (Shanghai) Co., Ltd..
(2) PCR amplification was performed according to the primers and templates listed in Table 6 to obtain the target fragments. The PCR reaction system and conditions were the same as 1.1 in Example 1.
(3) The amplified DNA fragments were recovered by gel cutting using a gel recovery kit, and positive single colonies were obtained by recombination, transformation, and plate screening using the multi-fragment recombination kit of NEB Company, and then the correct plasmids were obtained by picking up the  colony, shaking bacteria, and verifying via sequencing by Tsingke Biotechnology Co., Ltd..
Table 6 Primers required for construction of expression plasmid SL023
Figure PCTCN2022124823-appb-000008
3.2 Production of 6’-SL using strain SLIS026
3.2.1 Construction of 6’-SLE. coli strains
Competent cells were prepared on the basis of the gene knockout strain SLIS026, and the specific method was the same as 1.2.1. Then the following plasmid combinations: SL006+pET28a-neuB+SL023,  SL006+pET28a-neuB+SL023-Tet, SL006+pET28a-neuB+SL023-Tac, SL006+pTet-neuB+SL023, SL006+pTet-neuB+SL023-Tac, SL006+pTet-neuB+SL023-Tet 、SL006+pTac-neuB+SL023, SL006+pTac-neuB+SL023-Tac, SL006+pTac-neuB+SL023-Tet were transformed into SLIS026 competent cells, respectively, and the correct clones were screened on LB plates (kanamycin 50 μg/mL, chloramphenicol 25 μg/mL, ampicillin 100 μg/mL) to obtain a strain E. coli SLIS026-6SL carrying 6’-SL synthesis pathway.
3.2.2 Production of 6’-SL by SLIS026-6SL strain
(1) TB medium: 12g of trypsin, 24g of yeast extract, 4mL of glycerol, 2.31g KH 2PO 4, 12.54g K 2HPO 4, supplemented to 1000mL with deionized water, and finally divided into conical flasks at 100mL/bottle and wrapped well, sterilized at 121 ℃ for 30 min, and stored at room temperature.
(2) LB medium: 10 g of tryptone, 5 g of yeast extract, 10 g of NaCl, and 15 g of agar were weighed respectively, dissolved with distilled water and mixed well, and were supplemented to 1 L after the pH was adjusted to 7.2 with 1 mol/L NaOH, and finally divided into conical flasks and wrapped well, sterilized at 121 ℃ for 30 min, and stored at 4 ℃. The LB liquid was withour agar.
(3) 200g/L MgSO 4·7H 2O stock solution: 10g MgSO4·7H2O solution was weighed and added into deionized water, supplemented to 50mL after completely dissolved, then sterilized at 121℃ for 30 minutes, and stored at room temperature.
(4) Trace element stock solution: 54.4g/L ferric ammonium citrate, 9.8 g/L MnCl 2·4H 2O, 1.6 g/L CoCl 2·6H 2O, 1 g/L CuCl 2·2H 2O, 1.9 g/L H 3BO 3, 9 g/L ZnSO 4·7H 2O, 1.1 g/L Na 2MoO 4·2H 2O, 1.5 g/L Na 2SeO 3, 1.5 g/L NiSO 4·6H 2O. The trace element stock solution was prepared according to the above concentrations, sterilized at 121 ℃ for 30 min, and stored at 4 ℃ for future use.
(5) 1000 g/L glycerol: 1000g glycerol was weighed, supplemented to 1L with deionized water, then sterilized at 121℃ for 30 minutes, and stored at room temperature.
(6) 250 g/L lactose: 250 g of lactose was dissolved in deionized water  (dissolved by heating) , and supplemented to 1L, then sterilized at 121 ℃ for 30 min, and stored at room temperature.
(7) The strains were inoculated into 5 mL of LB medium (kanamycin 50 μg/mL, chloramphenicol 25 μg/mL) , and cultured at 37℃ and 250 rpm for 4 h. Then the seed liquid was inoculated into fresh TB medium at a ratio of seed liquid: medium = 1: 100, cultured at 37 ℃, 250 rpm to OD600 = 0.6-0.8, and IPTG was added to a final concentration of 0.1 mM and the medium was cultured under the condition of 25 ℃, 250 rpm for 15h for the inducible expression of protein.
(8) After the inducible expression of protein, 1mL of 200g/L MgSO 4·7H 2O (final concentration of 2g/L) , 100μl of trace element stock solution (final concentration of 0.1%) , 2mL of 1000g/L glycerol (final concentration of 20g/L) and 2 mL of 250 g/L lactose (final concentration of 5 g/L) were added to the shaking flask, and cultured at 30 ℃ and 250 rpm for 24 h, and then sampled to detect the content of SL.
(9) Means of sample treatment: 2-3 mL of the fermentation broth was taken, and the cells were broken by repeated freezing and thawing, placed into boiling water and boiled for 20 minutes after broken, and then centrifuged (4℃, 12000rpm for 5 minutes) to remove the precipitation and retain the supernatant, and passed through a 0.22μm filter membrane to detect the content of 6’-SL. The content of 6’-SL in each treatment at 24 h of fermentation is shown in Table 7, when the Tet promoter was used to regulate the expression of neuB, i. e., the combination of SL006+pTet-neuB+SL023, 6’-SL content is the highest, which could reach 2.5g/L. The SL detection results are shown in the accompanying drawings, Figure 11 shows the detection profile of 6’-SL standard, and Figure 12 shows the detection profile of 6’-SL fermentation broth.
Table 7 Detection results of 6’-SL content
Figure PCTCN2022124823-appb-000009
Figure PCTCN2022124823-appb-000010
Example 4 Production of 3’-SL using SLIS026 strain
4.1 Construction of expression plasmids related to 3’-SL synthesis
(1) Total gene synthesis was performed on gene SEQ ID NO: 63 encoding α-2, 3-sialyltransferase (3ST, GA040) with accession number AJC62560.1, which was synthesized and ligated into the pET28a vector by Sangon Biotech (Shanghai) Co., Ltd..
(2) PCR amplification was performed according to the primers and templates listed in Table 8 to obtain the target fragments. The PCR reaction system and conditions were the same as 1.1 in Example 1.
(3) The amplified DNA fragments were recovered by gel cutting using a gel recovery kit, and positive single colonies were obtained by recombination, transformation, and plate screening using the multi-fragment recombination kit of NEB Company, and then the correct plasmids were obtained by picking up the colony, shaking bacteria, and verifying via sequencing by Tsingke Biotechnology Co., Ltd..
Table 8. Primers required for construction of expression plasmid SL037
Figure PCTCN2022124823-appb-000011
4.2 Production of 3’-SL during fermentation
4.2.1 Construction of 3’-SLE. coli strains
Competent cells were prepared on the basis of the gene knockout strain SLIS026, the specific method was the same as 1.2.1, and then the plasmid combination SL006+pTet-neuB+SL037 was transformed into SLIS026 competent cells with reference to the plasmid combination with the best 6’-SL fermentation result, and the correct clones were screened on LB plates (kanamycin 50 μg/mL, chloramphenicol 25 μg/mL, ampicillin 100 μg/mL) to obtain a strain E. coli SLIS026-3SL carrying the 3’-SL synthesis pathway.
4.2.2 Production of 3’-SL by SLIS026-3SL strain
(1) The type and preparation of the medium required during the experiment are the same as 3.2.
(2) The strains were inoculated into 5 mL of LB medium (kanamycin 50 μg/mL, chloramphenicol 25 μg/mL) , and cultured at 37℃ and 250 rpm for 4 h. Then the seed liquid was inoculated into fresh TB medium at a ratio of seed liquid: medium = 1: 100, cultured at 37 ℃, 250 rpm to OD600 = 0.6-0.8, and IPTG was added to a final concentration of 0.1 mM and the medium was cultured under the condition of 25 ℃, 250 rpm for 15h for the inducible expression of protein.
(3) After the inducible expression of protein, 1mL of 200g/L MgSO 4·7H 2O (final concentration of 2g/L) , 1mL of trace element stock solution (final concentration of 0.1%) , 2mL of 1000g/L glycerol (final concentration of 20g/L) and 2 mL of 250 g/L lactose (final concentration of 5 g/L) were added to the shaking flask, and cultured at 30 ℃ and 250 rpm for 24 h, and then sampled to detect the content of SL.
(4) Means of sample treatment: Means of sample treatment were the same as 3.2. Quantitative detection of 3’-SL was carried out, and the results showed that the content of 3’-SL can reach 3g/L at 24 hours of fermentation. Figure 14 shows the detection profile of 3’-SL standard. Figure 15 shows the detection profile of 3’-SL fermentation broth.

Claims (11)

  1. A genetically engineered bacterium having an N-acetylneuraminic acid biosynthesis pathway, wherein the genetically engineered bacterium contains multiple copies of a gene neuB encoding a sialic acid synthase, and the gene neuB is initiated for expression by a strong promoter.
  2. The genetically engineered bacterium of claim 1, wherein an N-acetylneuraminic acid catabolic pathway in the genetically engineered bacterium is disabled; preferably:
    all or part of the genes in the N-acetylneuraminic acid catabolic pathway in the genetically engineered bacterium are knocked out; preferably one or more of a gene nanK encoding N-acetylmannosamine kinase, a gene nanE encoding N-acetylmannosamine-6-phosphate epimerase, and a gene nanA encoding N-acetylneuraminic acid aldolase are knocked out;
    and/or, a gene LacZ encoding a lactose operon beta-galactosidase in the genetically engineered bacterium is knocked out.
  3. The genetically engineered bacterium of claim 1, wherein the multiple copies are achieved by one or more means of replicon, multi-site integration in genome and insertion of an exogenous plasmid; preferably are achieved by means of insertion of an exogenous plasmid;
    and/or, the strong promoter is a Tet promoter.
  4. The genetically engineered bacterium of any one of claims 1 to 3, further comprising a gene neuB encoding sialic acid synthase, a gene slr1975 encoding N-acetylglucosamine 2-epimerase, a gene YqaB encoding N-acetylglucosamine-6-phosphate phosphatase, a gene Gna1 encoding glucosamine-6-phosphate acetyltransferase, a gene glmS encoding L-glutamine-D-fructose-6-phosphate transaminase, and a gene ppsA encoding phosphoenolpyruvate synthase; preferably the gene neuB, the gene slr1975, the gene YqaB, the gene Gna1, the gene glmS and the geneppsA are linked in tandem in a plasmid vector 1;
    wherein, the genes in the plasmid vector 1 preferably meet one or more of  the following conditions:
    the GenBank accession number of the gene neuB is AF305571;
    the GenBank accession number of the gene slr1975 is BAL35720;
    the gene YqaB is from the BL21 genome;
    the GenBank accession number of the gene Gna1 is NP_116637;
    the nucleic acid sequence of the gene glmS is set forth in SEQ ID NO: 66;
    the geneppsA is from the BL21 genome;
    preferably, the plasmid vector 1 is pACYCDuet.
  5. The genetically engineered bacterium of any one of claims 1 to 4, wherein, the genetically engineered bacterium further comprises a gene encoding N-acylneuraminic acid cytidylyltransferase and a gene encoding sialyltransferase;
    preferably, the gene encoding N-acylneuraminic acid cytidylyltransferase and the gene encoding sialyltransferase are linked in tandem in a plasmid vector 2; preferably the N-acylneuraminic acid cytidylyltransferase has the NCBI accession number WP_003512903.1, and preferably comprises the nucleic acid sequence set forth in SEQ ID NO: 61;
    the sialyltransferase is α-2, 6-sialyltransferase or α-2, 3-sialyltransferase, preferably the α-2, 6-sialyltransferase is an enzyme with NCBI accession number BAF91416, and preferably comprises the nucleic acid sequence set forth in SEQ ID NO: 62; the α-2, 3-sialyltransferase is an enzyme with NCBI accession number AJC62560.1, and preferably comprises the nucleic acid sequence set forth in SEQ ID NO: 63; preferably, the plasmid vector 2 is pET28a.
  6. The genetically engineered bacterium of any one of claims 1 to 5, wherein the starting bacterium is Escherichia coli BL21 (DE3) .
  7. A method for producing N-acetylneuraminic acid, wherein the N-acetylneuraminic acid is obtained by culturing the genetically engineered bacterium of any one of claims 1 to 4, and 6.
  8. A method for producing sialyllactose by fermentation, comprising using the genetically engineered bacterium of any one of claims 1 to 6, adding lactose into a fermentation medium for fermentation, and extracting the sialyllactose from the fermentation broth; and
    optionally, obtaining 3’-sialyllactose when the genetically engineered bacterium comprises a α-2, 3-sialyltransferase gene; obtaining 6’-sialyllactose when the genetically engineered bacterium comprises α-2, 6-sialyltransferase gene.
  9. The method of claim 8, wherein the fermentation medium is a TB medium; the TB medium comprises 12 g/L trypsin, 24 g/L yeast extract, 4 mL/L glycerol, 2.31 g/L KH 2PO 4 and 12.54 g/L K 2HPO 4.
  10. The method of claim 8, comprising inducing culture with IPTG when cultured to OD value of 0.6 to 0.8;
    and/or, supplementing 2 g/L MgSO 4·7H 2O, 20 g/L glycerol, 1 mL/L trace element stock solution and 5g/L lactose after the induction culture; the trace element stock solution preferably comprises 54.4 g/L ferric ammonium citrate, 9.8 g/L MnCl 2·4H 2O, 1.6 g/L CoCl 2·6H 2O, 1 g/L CuCl 2·2H 2O, 1.9 g/LH 3BO 3, 9 g/L ZnSO 4·7H 2O, 1.1 g/L Na 2MoO 4·2H 2O, 1.5 g/L Na 2SeO 3 and 1.5 g/L NiSO 4·6H 2O.
  11. The method of any one of claims 8 to 10, wherein the fermentation culture condition is culture with shaking at 250 rpm at 30℃.
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